1. Field of the Invention
The present invention generally relates to engines, and more specifically to a uniquely configured crankshaft protruding member for an outboard motor engine. The crankshaft protruding member can support a flywheel and a drive pulley of the engine and allow the overall weight, number of parts, and general complexity of the engine to be reduced.
2. Description of the Related Art
A conventional outboard motor engine is disclosed in Japanese Patent Document No. JP-A-2004-147438 (hereinafter “JP '438”). The outboard motor engine of JP '438 is a four-stroke engine with a crankshaft that extends vertically through the engine with one end being connected to a propeller side, i.e. a driven member side, and another end being connected to a flywheel. A protruding member extends axially outward from the flywheel end of the crankshaft and rotates with the crankshaft. The protruding member has a protruding end side and a base side. The flywheel is fixed on the protruding end side of the protruding member by a tightening member. Further, a drive pulley is axially aligned with the crankshaft and fixed thereto on the base side of the protruding member. An endless drive belt engages the drive pulley in order to link a valve drive mechanism to the protruding member. The drive pulley is formed separately from the protruding member and is fitted and fixed on an outer face of the protruding member.
While the engine is running, intake and exhaust cam shafts of the valve drive mechanism are used to control the intake and exhausting of air. The valve drive mechanism is linked to a driven pulley of the valve drive mechanism via the drive pulley on the crankshaft and an endless drive belt. Rotational fluctuation of the crankshaft is suppressed and smooth drive is ensured by use of the flywheel, which is linked to the crankshaft. Therefore, a smooth drive force is transmitted through the crankshaft to the propeller side to drive the propeller.
JP '438 further teaches that screw holes are formed on the protruding member to fasten bolts of the tightening member. However, if it is required that the strength of the bolts be increased, which typically requires that the outer diameter of the bolts likewise increase, the screw holes formed in the protruding member—and the protruding member itself, and components thereof—must also be increased in size.
In general, the protruding member and the drive pulley fixed thereto are located in the same axial position along the protruding member. Thus, if the size or outer diameter of the protruding member increases, the size of the inner diameter of a drive pulley fitted on the outer face of the protruding member must also be enlarged to properly match the outer diameter of the protruding member.
Additionally, in some cases, the external size of the drive pulley must also be increased in order to achieve a larger inner diameter. In such a case, in order to preserve the speed reduction ratio between the drive pulley and a driven pulley linked to the drive pulley via the endless drive belt to a specific value (such as ½), the size of the outer diameter of the driven pulley must also be enlarged accordingly.
As a result, various parts of the engine, as well as the engine in general, become larger and heavier. This is undesirable, especially for an outboard motor engine.
Additionally, in the above-described engine, the drive pulley is typically formed separately from the protruding member. Thus, a further drawback of such an engine is that it requires an increased number of components which only complicates the structure of the engine.